What IDMT Means
An inverse definite minimum time (IDMT) overcurrent relay trips faster the larger the fault current is. Small overloads take a long time to trip and large short circuits trip quickly, which matches how much stress the equipment actually experiences.
The Standard Curve Families
- Normal (standard) inverse - the general-purpose choice.
- Very inverse - trips more steeply as current rises, useful where fault current changes a lot with fault location.
- Extremely inverse - even steeper, often used to coordinate with fuses and for equipment with high inrush.
- Long-time inverse - for slow overload-type protection.
Settings That Shape the Curve
- Pickup (plug setting) - the current level above which the relay starts to operate.
- Time multiplier setting - scales the operating time up or down along the same curve shape.
Coordination
The goal of grading is that the relay closest to the fault operates first, with upstream relays acting as backup after a time margin. That requires a proper coordination study rather than default settings. See protection relay types.
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